A well drilled into a hydrocarbon-bearing subsurface formation, during an initial post-completion stage, commonly produces crude oil and/or natural gas without artificial stimulation, because pre-existing formation pressure is effective to force the crude, oil and/or natural gas out of the formation into the well bore, and up the production tubing of the well. However, the formation pressure will gradually dissipate as more hydrocarbons are produced, and will eventually become too low to force further hydrocarbons up the well. At this stage, the well must be stimulated by artificial means to induce additional production, or else the well must be capped off and abandoned. This is a particular problem in gas wells drilled into “tight” formations, i.e., where natural gas is present in subsurface materials having inherently low porosities, such as sandstone, limestone, shale, and coal seams (e.g., coal bed methane wells).
Despite the fact that very large quantities of hydrocarbons may still be present in the formation, it has in the past been common practice to abandon wells that will no longer produce hydrocarbons under natural pressure, where the value of stimulated production would not justify the cost of stimulation. In other cases, where stimulation was at least initially a viable option, wells have been stimulated for a period of time and later abandoned when continued stimulation became uneconomical, even though considerable hydrocarbon reserves remained in the formation. With recent dramatic increases in market prices for crude oil and natural gas, well stimulation has become viable in many situations where it would previously have been economically unsustainable.
There are numerous known techniques and processes for stimulating production in low-production wells or in “dead” wells that have ceased flowing naturally. One widely-used method is hydraulic fracturing (or “fraccing”). In this method, a fracturing fluid (or “frac, fluid”) is injected under pressure into the subsurface formation. Frac fluids are specially-engineered fluids containing substantial quantities of proppants, which are very small, very hard, and preferably spherical particles. The proppants may be naturally formed (e.g., graded sand particles) or manufactured (e.g., ceramic materials; sintered bauxite). The frac fluid may be in a liquid form (often with a hydrocarbon base, such as diesel fuel), but may also be in gel form to enhance the fluids ability to hold proppants in a uniformly-dispersed suspension. Frac fluids commonly contain a variety of chemical additives to achieve desired characteristics.
The frac fluid is forced under pressure into cracks and fissures in the hydrocarbon-bearing formation, and the resulting hydraulic pressure induced within the formation materials widens existing cracks and fissures and also creates new ones. When the frac fluid pressure is relieved, the liquid or gel phase of the frac fluid flows out of the formation, but the proppants remain in the widened or newly-formed cracks and fissures, forming a filler material of comparatively high permeability that is strong enough to withstand geologic pressures so as to prop the cracks and fissures open. Once the frac fluid has drained away, liquid and/or gaseous hydrocarbons can migrate through the spaces between the proppant particles and into the well bore, from which they may be recovered using known techniques.
Another known well stimulation method is acidizing (also known as “acid fracturing”). In this method, an acid or acid blend is pumped into a subsurface formation as a means for cleaning but extraneous or deleterious materials from the fissures in the formation, thus enhancing the formation's permeability. Hydrochloric acid is perhaps most commonly as the base acid, although other acids including acetic, formic, or hydrofluoric acid may be used depending on the circumstances.
Another relatively new and more effective system and method for stimulating production in oil and gas wells is described in U.S. Pat. No. 7,775,281 entitled METHOD AND APPARATUS FOR STIMULATING PRODUCTION FROM OIL AND GAS WELLS BY FREEZE-THAW CYCLING and pending U.S. Patent Application Publication No. 2010/0263874 entitled METHOD AND APPARATUS FOR FREEZE-THAW WELL STIMULATION USING ORIFICED REFRIGERATION TUBING, both of which are incorporate herein by reference in their entireties. This new system and method fractures the subsurface formation by freezing a water-containing zone within the formation in the vicinity of a well, thereby generating expansive pressures which expand or created cracks and fissures in the formation. The frozen zone is then allowed to thaw. This freeze-thaw process causes rock particles in existing cracks and fissures to become dislodged and reoriented therewithin, and also causes new or additional rock particles to become disposed within both existing and newly formed cracks and fissures. The particles present in the cracks and fissures act as natural proppants to help keep the cracks and fissures open, thereby facilitating the flow of fluids from the formation into the well after the formation has thawed. Freeze-thaw fracturing enables recovery of higher percentages of non-naturally-flowing hydrocarbons from low-permeability formations than has been possible using previously known stimulation methods.
Prior to introducing a stimulation method, it is know to prepare the well by initiating cracks or instability in the well walls to better respond to the stimulation methods. This preparation has previously been accomplished using, a charge or other mechanical means, e.g., vibration hammer, to create a stress crack. These prior art preparation methods either include moving parts, which introduced inefficiencies into the process and/or they would damage wells having pre-existing slotted or perforated liners. Accordingly, there is a need in the art for an improved system and method for pre-treating a well bore in order to improve stimulation methods such as those discussed above.